The present invention relates to a device and method for controlling input of a rechargeable battery.
In recent years, rechargeable batteries have been combined with fuel cells, solar cells, or power generators to form power supply systems. A power generator is driven by natural power, such as wind power or water power, or by artificial power, such as power generated by an internal combustion engine. A power supply system using a rechargeable battery stores excess power in the rechargeable battery to improve energy efficiency.
An example of a power supply system is a hybrid electric vehicle (HEV) using an engine and a motor as its power sources. When the engine outputs more power than necessary to drive the vehicle, the HEV drives its generator using the excessive power to charge the rechargeable battery. When the vehicle is braking or decelerating, the HEV drives the motor with the vehicle wheels and charges the rechargeable battery using the motor as a power generator. When the engine outputs less power than necessary, the HEV compensates for the lack of power by discharging the rechargeable battery and driving the motor.
In this way, the HEV accumulates energy in the rechargeable battery. Conventional automobiles release such energy into the atmosphere as heat. The energy efficiency of an HEV is higher than the energy efficiency of a conventional automobile. Thus, the HEV greatly improves fuel efficiency as compared with conventional automobiles.
To efficiently charge excess power into the rechargeable battery of the HEV, the rechargeable battery is controlled in a manner that its state of charge (SOC) does not reach 100%. To drive the motor when necessary, the rechargeable battery is also controlled in a manner that its SOC does not reach zero. More specifically, the rechargeable battery is normally controlled in a manner that the SOC varies within a range of 20 to 80%.
A rechargeable battery installed in the HEV or other power supply systems may be formed by connecting a plurality of battery cells (cells) in series. In this case, the rechargeable battery may be overcharged due to variations in the capacities of the battery cells. When a battery cell having a small capacity is overcharged, gas is generated in the battery cell and the internal pressure of the battery cell increases. When the battery cell is continuously overcharged, a safety valve is operated. This releases the internal gas together with electrolyte. As a result, the life of the overall rechargeable battery is shortened.
To prevent the life of the rechargeable battery from being shortened by overcharging, for example, Japanese Laid-Open Patent Publication No. 2005-137091 describes an input control method for limiting the power that is supplied to charge a rechargeable battery installed in an HEV. When braking is performed while the vehicle is driving at a high speed, a large power is generated and the rechargeable battery may thus be easily overcharged. The method described in the above publication controls the input of the rechargeable battery to reduce the amount of the charging power as the time for which the vehicle drives at a high speed becomes longer.
However, the method described in the publication limits the charging power amount in the same manner when the vehicle is traveling at a high speed irrespective of whether brakes are applied normally or suddenly. Even when the vehicle is braking gradually and not generating excessively high power, the method limits the charging power amount in the same manner as when braking is suddenly performed. This lowers the energy recovering efficiency.